1. Field of the Invention
This invention concerns phthalocyanines, and relates in particular to processes for the preparation of certain metal phthalocyanines which may be of use as semiconductor and/or electrochromic materials.
2. Description of the Prior Art
Phthalocyanines--also referred to as benzoporphyrins--have been well-known for many years as powerful colouring agents useful in the preparation of dyestuffs. Most of these materials, whether mono- or diphthalocyanines, and both substituted and unsubstituted, are in the form of complexes with metals. Certain of these complexes--expecially the diphthalocyanines of tri- and tetravalent metals, principally the rare earth metals (the 15 elements from lanthanum to lutetium; particularly europium, terbium and lutetium), and associated metals such as yttrium--are well-known for their semiconductor and/or electrochromic properties. A material is said to be electrochromic if it changes colour in response to changes in the magnitude or direction of the electric potential applied across it; lutetium diphthalocyanine, a material to which a considerable amount of study has been devoted, can be persuaded (suitably arranged in an appropriate electrolytic cell) to display the colours from violet and deep blue, through various shades of green, to orange and red. Materials such as this have obvious possibilities in colour display systems of many sorts.
The diphthalocyanines are of the empirical general formulae EQU (PC).sub.2 H.M.sup.III (Ia)
or EQU (PC).sub.2.M.sup.IV (Ib)
wherein M.sup.III and M.sup.IV represent respectively the tri- and tetravalent metal, and "PC" represents the phthalocyanine ring system. Phthalocyanine itself is shown in full in Formula II of FIG. 1. H--as normal--represents hydrogen.
The structure of the compounds Ia and/or Ib is such that a single metal atom is "sandwiched" between two opposing phthalocyanine ring systems, and Formula III of FIG. 2 shows a simplified, general structure for complexes of this kind where M is the metal, "BI" represents the individual benzoisoindole ring systems making up the phthalocyanine system, and for clarity the bonds joining the two phthalocyanine systems to the metal atom have been omitted.
Metal diphthalocyanines are presently prepared by a process in which an organic derivative of the metal (an acetate, for example) is reacted, at about 300.degree. C. in a sealed tube, with an appropriate phthalonitrile. This process is itself unsatisfactory, not the least because the reaction product is a mixture including various by-products, and the necessary purification (by chromatography) is difficult and time-consuming. The invention seeks to provide a novel process for the preparation of metal diphthalocyanines, which process avoids this difficulty.
Another problem connected with metal diphthalocyanines concerns their mode of use. The compounds have to be applied to a surface, and in practice the only convenient way to effect this is by vapour deposition. Unfortunately, because of their high molecular weight the compounds are extremely difficult to evaporate and then deposit as a thin film, and even where evaporation can be achieved the temperatures required--of the order of 500.degree. C. and higher--are such as to result in not inconsiderable decomposition. The invention seeks to provide a novel method for the thin film application of metal diphthalocyanines, which method avoids these difficulties.